Fix Nim 2.0.0 compile error caused by _ variables

[fukusuket]

Hello, Thank you so much for maintaining tool :) I created a fix PR for the issue below.

• Fix Fail to build with nim 2.0.0 #105

In Nim 2.0.0 released on August 1st, it seems that _ can no longer be used as a variable due to the following changes.

• document general use of _, error message, fixes nim-lang/Nim#21584
• Document underscore nim-lang/Nim#20687
• Improve error message of generic declarations using [_] as type, as undeclared identifier _ compiler error is hard to understand nim-lang/Nim#21435

Currently, only the macOS code had _ usage, so I replaced it with x.

For this pull request, I have confirmed following:

• Successful compilation with Nim 2.0.0 and 1.6.14
• README some sample code works

I would appreciate it if you could review.

Regards,

[treeform]

Thanks this is great! Got to get 2.0 ready!

[fukusuket]

Thank you so much for quick review :)